The present invention relates to switching regulator circuits. More particularly, the present invention relates to circuits and methods for regulating output voltage based on approximations of load current.
The purpose of a voltage regulator is to provide a predetermined and substantially constant output voltage to a load from a poorly-specified and fluctuating voltage source. One type of voltage regulator commonly used to accomplish this task is a switching voltage regulator. Switching voltage regulators are typically arranged to have a switching element, such as a power transistor, coupled between the voltage source and the load. The switching regulator controls the voltage across the load by turning the switching element ON and OFF so current passes through it and into an inductor in the form of discrete current pulses. The inductor and an output capacitor then convert these current pulses into a substantially steady load current so that the load voltage is regulated.
To generate a stream of current pulses, switching regulators include control circuitry that commands the switching element ON and OFF. The duty cycle of the switching element (i.e., the amount of time the switching element is ON compared to the period of an ON/OFF cycle), which controls the flow of current into the load, can be varied by a variety of methods. Pulse width modulation (PWM), for example, can be used to vary the duty cycle of the switching element by fixing the pulse stream frequency and varying the ON (or OFF) time of each current pulse. Another commonly employed technique is pulse frequency modulation (PFM), in which the ON or OFF time of each current pulse is fixed and the frequency of the pulse stream is varied.
The abrupt switching and accompanying discharge of energy stored in inductive filter elements typically results in undesirably large ripple voltages at the output of the regulator. This ripple voltage is generally proportional to the product of the equivalent series resistance (ESR) of the filter capacitors and the inductor current. In many instances, the peak inductor current rises to relatively large levels due to the size of the inductor required to accommodate worst-case operating scenarios of the regulator. As a result, a significant amount of ripple may be present on the regulator output.
Ripple voltages may be minimized by using “current mode” type switching regulators. Rather than relying directly on output voltage for control, current-mode switching regulators use a signal indicative of switch current to provide regulation information. For example, a current-mode switching regulator may use peak switch current as the criteria for determining when to modify the switching element duty cycle. Current mode switching regulators often incorporate special circuits designed to reduce current flow and ripple depending on the load. For example, a current-mode switching regulator may use an error amplifier to control the peak, average, or minimum values of regulator output current (i.e., inductor or load current) based on the difference between the output voltage and the desired ideal regulated voltage.
The method of using a single peak current threshold for current mode control, however, may not be satisfactory under a variety of commonly encountered conditions (e.g., at start-up or under low output load currents). Under such conditions, a feedback loop using an error amplifier may not be stable. To avoid feedback instability, some switching regulators operate in one of two modes depending on the magnitude of an error signal, with each mode having a distinct peak current threshold. The use of two distinct peak current thresholds, however, may lead to undesirable subharmonic oscillations with the output current oscillating between the two distinct threshold levels. To avoid this problem, only one distinct peak level is used during normal operation. The dual thresholds are used only during startup or in cases where the output voltage is severely out of regulation. An example of this type of regulator is the LTC1174, manufactured by Linear Technology Corporation, Milpitas, Calif.
Other commercially available switching regulators, such as the MAX774/5/6 series manufactured by Maxim Integrated Products Inc., Sunnyvale, Calif., may also use two distinct peak current threshold levels for current control, albeit differently. With light loads, the first two switch cycles use the lower peak current threshold level in an attempt to regulate the output voltage. If regulation of the output voltage is achieved within two switch cycles, the regulator continues to operate with the lower peak current threshold level. If regulation is not achieved within two cycles, the regulator begins to operate with the higher peak current threshold level. This technique, however, does not always avoid instability in the feedback loop. It also exhibits poor pulse response because the initial pulses are set at low current levels.
In view of the foregoing, it would be desirable to provide switching regulator circuits that have the benefits of current mode control, such as low ripple, and yet provide efficient voltage regulation under a variety of input voltage and output load conditions.